It is well known to use flywheels to store mechanical kinetic energy. The amount of energy stored depends on the mass of the flywheel and its speed of rotation. The kinetic energy of the flywheel is proportional to the square of its angular velocity. However, the amount of energy that may be stored in a given flywheel is dependent upon the robustness of the flywheel assembly and how it responds to the stresses experienced at high rotational speeds.
It is preferable to form flywheel assemblies from materials which are of low density and high specific strength. The materials also need to be extremely strong. For use in, for example, hybrid vehicles or uninterruptable power supplies, energy storage flywheel assemblies need to operate at extremely high speeds, greater than 10,000 rev./min or even more than 50,000 or 100,000 rev./min. Accordingly, there is considerable demand for flywheel assemblies that are able to operate reliably at speeds of these orders.
WO2010/020806 (filed by the present applicant) discloses a flywheel assembly in which the rotor is connected to a composite end cap. The end cap is mounted on a shaft.